1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, in which a laser beam is used to dice a substrate having semiconductor layers formed thereon, with gallium contained in at least one of the substrate and the semiconductor layers.
2. Description of Related Art
During the manufacture of light-emitting elements, which are an example of gallium-based semiconductor devices, circuits have to be formed on a wafer used as a substrate and light-emitting elements have to be produced by partitioning the substrate vertically and horizontally.
FIG. 5A-C illustrates a method for manufacturing light-emitting elements, which are an example of conventional semiconductor devices utilizing substrates on which semiconductor layers are formed, containing gallium.
A substrate 1 is formed from sapphire, a p-type semiconductor layer 3 and an n-type semiconductor layer 2 composed of gallium nitride are formed on the substrate 1 by epitaxial growth, using metal-organic chemical vapor deposition (MOCVD: metal-organic chemical vapor deposition), portions of the p-type semiconductor layer 3 are removed by reactive ion etching (RIE) to expose the n-type semiconductor layer 2, n-type electrodes 4 are formed on the exposed n-type semiconductor layer 2, and a reflective film 5b composed of rhodium and, on top of it, a p-type electrode 5a composed of gold, are formed on the p-type semiconductor layer 3 (FIG. 5A). The reflective film 5b reflects light generated by the p-type semiconductor layer 3 downward.
Next, grooves 10 are formed (FIG. 5B) on the substrate 1 using a dicing saw (not shown) and individual substrates 1a are obtained by partitioning along the grooves 10 (FIG. 5C).
In case of semiconductor devices utilizing gallium-arsenic, gallium-phosphorus, and similar substrates, the hardness of the substrates is not very high and partitioning is performed by dicing with a dicing saw. However, in case of substrates utilizing sapphire and gallium nitride, which have a high hardness and are difficult to dice using a dicing saw alone, a technique has been adopted, wherein grooves are formed with a dicing saw along the partitioning locations and the substrate is partitioned by a mechanical action using the grooves as boundaries.
Due to the high hardness of sapphire and gallium nitride, from which substrates in such conventional manufacturing methods are formed, the formation of the grooves used to be a time-consuming task and the blades of the saws used for dicing were heated to elevated temperatures and subjected to intense wear and tear, resulting in a high frequency of replacement and high costs. In addition, the high hardness of the substrates made it impossible to form deep grooves, and when partitioning was performed by splitting along shallow grooves, the cleavage surfaces were not flat and splitting tended to produce non-uniform fracture surfaces. FIG. 6A-B is an example of a semiconductor device with non-uniform fracture surfaces, wherein FIG. 6A is an overall view and FIG. 6B is an enlarged view of FIG. 6A. The problem was that when cleavage surfaces were non-uniform fracture surfaces, splitting could affect the substrates of the adjacent semiconductor devices.
To address this problem, proposals have been made to form the grooves using a laser beam from a YAG (Yttrium Aluminum Garnet) laser, etc. A laser beam can produce elevated temperatures locally and thus can form deep grooves quickly, thereby permitting partitioning of a semiconductor device substrate into individual pieces within a short processing time.
JP H11-163403A (paragraphs Nos. 0038-0043, FIG. 1) has disclosed a technology, in which one side of a substrate for nitride semiconductor devices is irradiated with a laser beam along vertical and horizontal transects to form scribe lines, whereupon grooves are formed on the other side with a dicer so as to match the scribe lines formed on the opposite side and the substrate for nitride semiconductor devices is partitioned.
In addition, JP H11-177137A (paragraphs Nos. 0042-0045, FIG. 1) has disclosed a technology, in which grooves are formed with a dicer on one side of a substrate for nitride semiconductor devices, cleavage grooves are then formed by irradiating the grooves with a laser beam, and the substrate for nitride semiconductor devices is partitioned along the cleavage grooves.
In addition, JP 2001-284292 (paragraphs Nos. 0027-0034, FIG. 1) has disclosed a method, wherein partitioning grooves are formed by irradiating the surface of the side, on which the semiconductor is formed of a semiconductor wafer, with laser light, and then partitioning is performed by subjecting the semiconductor wafer to thermal shock by bringing it in contact with liquid nitrogen so as to induce cracking along the partitioning grooves under the action of thermal stress.
However, the problem with the conventional method, in which grooves were formed by locally heating a substrate to an elevated temperature of 1000° C. or more by irradiation with a laser beam along partitioning locations and sublimating the gallium-containing compounds utilized in the substrate and the semiconductor layers deposited on the substrate, was that some of the sublimated gallium-containing compounds converted into gallium oxide and other gallium compounds (hereafter called “gallium compounds”) and adhered to the substrate and to the electrodes.
The adhesion of the deposits of gallium compounds to the electrode 5a formed on the substrate caused various problems, such as decreased electrical conduction and reduced wire bond strength when bonding the electrodes of the semiconductor device to other electrodes and in case of wire bonding to other electrodes.
Another problem was that in the case where resin sealing is performed with such deposits still adhered to the substrates, gaps appeared between the substrate and the resin, which allowed moisture to penetrate inside and, upon heating, caused peeling due to interface contamination by the deposits.